It has become increasingly useful to enclose liquids in plastic containers to separate the liquids from supporting walls in order to utilize economical lightweight support materials which can be easily attacked by the liquids, but which otherwise have the strength or stiffness to support the weight of the liquid. Examples of such plastic containers include bag liners for metal or corregated paper containers, medical liquid liners used with or without support, or liners for holding supplies of liquid in space craft. One problem which often occurs in these uses is how to vent gases from the plastic containers or admit them into the containers where they may be used or consumed without attendant problems of sealing gas vents into the containers that will not leak the liquid around the seals which attach the vents to the containers or leak liquid from the vents themselves. The problems of leaking can become particularly severe where the liquids are strong electrolytes, such as aqueous caustic alkali solution, concentrated salt solutions, aqueous acids, or concentrated surfactant solutions. The plastic containers themselves can be manufactured to resist the electrolytes which may penetrate into the plastic to a small extent, but not cause the plastic to disintegrate for a long time. The plastic, such as polyethylene or polypropylene, may be layered with a support film of a different material, such as polyamide or polyester, which will support the polyolefin layer and aid in blocking attack by the electrolyte upon the polyolefin layers. The joint areas of a gas-venting seal attached to a container, however, may be degraded or rendered less effective by the electrolyte and generally tend to leak electrolytes outside the container. Although electrolytes are difficult to contain, the invention also has utility against other liquids.